1. Field of the Invention
The present invention relates to electrode heat cutout devices for fluorescent lamps, and, more particularly, to an electrode heat cutout switch mechanism employing a heat-responsive cutout switch connected to one lamp electrode lead and a resistive heater connected to the other lamp electrode lead.
2. Description of the Prior Art
Certain types of fluorescent lamps, e.g., rapid start type, are provided with cathode heating current for heating the cathode to electron-emitting temperature, so that the lamps start quickly without damaging the electron-emitting material deposited on the electrodes. This electrode heating consumes about 11/2 to 2 watts of electrical power per electrode during normal lamp operation. While the lamps are operating, "hot spots" form on the electrodes and can provide adequate electron emission without the need for continuing to supply heating current through the electrode. Therefore, about 3-4 watts of electrical energy per lamp are used to heat the lamp electrodes during operation rather than contributing to light output, resulting in the unproductive use of a considerable amount of energy, particularly in lighting systems in large buildings having hundreds or even thousands of fluorescent lamps. One prior art approach to electrode heat cutout is described in U.S. Pat. No. 4,517,493 issued May 14, 1985 to Dembowski et al and assigned to the assignee of the present case. The Dembowski et al patent discloses a rapid start fluorescent lamp provided with a pair of cathode mounts each having a pair of lead-in wires for cathode heating current, and a thermal switch mounted inside the lamp envelope to turn off the heating current after sufficient initial cathode heating and during operation of the lamps. A third lead-in wire is provided in each mount and is used to bypass the thermal switch for heating the cathodes to activate the emission mix during manufacture. After activation of the cathode, the third lead-in wire is not used, and the selected pairs of lead-in wires are connected to terminals of the lamp's end cap bases. In a lamp configuration such as shown in Dembowski et al, the insertion of the bimetal switch into the lamp envelope adds additional manufacturing steps to the lamp manufacturing. Another prior art starting switch for electric discharge lamps is disclosed in U.S. Pat. No. 2,462,335, issued Feb. 22, 1949 to B. R. Reinhardt and assigned to the assignee of the present case. The Reinhardt patent describes a switch mechanism in which two bimetal elements having separate thermal characteristics are employed to provide a fast-acting and slow-acting switch to enable both hot and cold lamp restart. In the Reinhardt patent, the switch mechanism is designed so that the slow-acting component will be heated indirectly by heat from the lamp discharge, and the fast-acting component is heated by a heating circuit. This complexity makes the switch arrangement of Reinhardt less attractive due to the number of components necessary to assemble in the manufacturing of the lamp.